LAST March Helen Lang had her fourth child, Catherine. It was a shock for her and her husband Chas and his family, who were convinced the new arrival would be another boy. Chas is one of three brothers, and between them they have produced six sons. Catherine is the first girl on Chas’s side of the family for three generations.
The family’s propensity for boys could admittedly be down to chance alone – if enough people toss a coin enough times, someone’s eventually going to get ten heads in a row. But could it also be something in the Lang genes? Something that Catherine’s genes managed to overcome?
Most of us think that the sex of our offspring is a matter of chance, a random process with an equal likelihood of producing a boy or a girl. But scientists believe that Mother Nature doesn’t simply toss a coin. And it isn’t just odd cases like that of Chas and his brothers that have convinced them. Records show that, worldwide, slightly more males are born than females – 106 boys for every 100 girls, in fact. Then there are all sorts of other strange findings. For example, high-status men such as presidents and lords tend to produce sons, but those who work as divers, test pilots, clergymen and sawmill workers have more daughters. More boys are born after unseasonably hot weather. Older fathers and those under stress produce more daughters. And, during and after each world war there was a glut of baby boys.
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It looks as though under certain circumstances some men are more likely to have sons while others have more daughters. For several decades, researchers have been trying to work out exactly what’s going on. But the picture is confused. Some studies are contradictory, and there’s no coherent theory that encompasses all the research findings.
But perhaps they’ve been looking for explanations in the wrong place. Maybe it’s not fathers who control the sex of their children. Maybe it’s mothers. That’s what Valerie Grant from the University of Auckland thinks. She’s found that she can predict whether a woman is more likely to have a boy or girl just by using a personality test. Dominant women have more testosterone, claims Grant, and that’s why they produce more sons.
She’s putting female biology at the centre of the debate about whether the sex of our children is a matter of chance or not. And it looks as though she may be onto something. Recent research has strengthened the link between testosterone and sons. And another study published earlier this year suggests that biologically fit mothers have more than their fair share of sons. Could research into sex determination benefit from being a bit less…well, sexist?
The idea that humans might somehow control the sex of their offspring isn’t as odd as it might first appear. Many animals do it, producing young of the sex that will do best under the prevailing conditions. And it’s usually the mother that takes control. In reptiles, environmental factors are often key. Female alligators, for example, produce sons if they lay their eggs in a relatively warm place and daughters if the incubation site is cooler. Insects such as ants, bees and wasps have an even more foolproof method: eggs that mothers allow to be fertilised become daughters, the rest become sons.
Humans may simply be doing what comes naturally to many other animals. But there’s a problem. Our sex is determined by our genetic make-up: if your 23rd pair of chromosomes is XX, you are female; if it’s XY, you’re male. Human embryos inherit an X from their mother and either another X or a Y from their father. And, in theory at least, men produce equal numbers of sperm carrying X and Y chromosomes. So on the face of it, we have no scope to affect the sex of our offspring.
Yet it’s clear that environmental factors can disrupt the balance. For example, some pollutants seem to selectively damage sperm carrying either the X or Y chromosome. Men exposed to high levels of radiation at the Sellafield nuclear power plant in Cumbria have been found to father 40 per cent more sons than daughters. And only this year, a study showed that parents who smoke more than 20 cigarettes a day produce boys 45 per cent of the time, compared with 55 per cent for those who don’t smoke at all.
Sometimes the environmental influence isn’t so clear, as with the finding that southern Europeans have more boys than northerners do. Victor Grech from St Luke’s Hospital in Malta, who made the discovery two years ago, suggested that warm weather boosts production of Y-carrying sperm. But in April he reported that in north America the trend was reversed, with more boys born in Canada, and fewer the further south you go. “[This] puts a spanner in the works,” says Grech. “Temperature does not play a role.”
Still, some environments undoubtedly favour boys and others girls. But the question remains: can parents actively, albeit unconsciously, exert control over their offspring’s sex? In theory, such an ability could evolve and spread through the human race if any genes that made sex manipulation possible gave the people who carried them an edge over those who didn’t. So what could be the survival advantage?
Robert Trivers from Rutgers University, New Jersey, and Dan Willard from Harvard University tried to explain the evolution of sex-ratio skewing in mammals three decades ago. Their hypothesis, which remains the most influential in the field, states that parents have offspring of the sex that is likely to produce the greatest number of grandchildren. In polygamous species, a male can in theory father children by more than his fair share of females, but only if he can outcompete rival suitors. So if you can sire strong, healthy, attractive children, better make them boys because they raise your chances of having many grandchildren. But if you can’t, then you had better play it safe with daughters, who are likely to produce at least some grandchildren.
Although human societies tend to be underpinned by monogamy, plenty of polygamy goes on too, so you’d think we would be subject to the Trivers-Willard effect. If true, there are three ways it might happen. First, high-quality males might produce more Y-carrying sperm, and weaker males more X sperm. Nobody knows how this might happen, but one study did find that men with only daughters had 70 per cent X sperm. That suggests that at least some of the single-sex dynasties like the Langs might be caused by some real biological effect, rather than just a statistical fluke. Another possibility is that embryos of one or the other sex are more likely to miscarry. There’s evidence that this happens in some other mammals but nobody knows whether selective abortion happens naturally in humans. Medical technology has, however, made it an option in recent years (see “Sexploitation”, below).
A third way that the sex of offspring could be skewed is if X and Y sperm have different success rates at fertilising eggs. There are various ways this might happen, but one factor thought to be important is the timing of conception during the menstrual cycle due to varying hormone levels. Studies suggest that conceptions produce more boys if they occur outside the woman’s peak fertility, which is before or after the mid-point of the cycle. This could explain the high proportion of sons born after the world wars and early in marriages, because frequent sex means fertilisation is likely to occur earlier in the cycle.
So far, so good. But some researchers think we’re still not seeing the whole picture. John Lazarus from the University of Newcastle-upon-Tyne has recently reviewed the research in this field and has found that of the 50 or so studies in humans that consider status and sex of offspring, virtually all focus on the father. But, he says, when you look at possible mechanisms you see that mothers could play an equally important role. “It takes two to have sex,” he points out. And if a baby’s sex depends on what point during the menstrual cycle it was conceived “then that’s a female effect”.
Anthropologist Boguslaw Pawlowski from the University of Wroclaw in Poland agrees. “I think the mother is more important,” he says. Earlier this year, Pawlowski and colleague Elzbieta Cieplak published findings from a study that they believe show a direct link between a mother’s biological “fitness” and the sex of her baby (Medical Hypotheses, vol 58, p 15). They took the birthweight of a mother’s first baby as an indicator of her physical condition and then looked to see if those who had previously had a good-sized baby were more likely to give birth to a boy next time around. For the 227 local women in their study, this proved to be the case.
On closer inspection, however, the results were only significant if the firstborn was a girl. Then the bias was impressive: 64 per cent of mothers whose first daughter was below average weight had a girl the next time around, while 67 per cent of those who had a heavier-than-average first daughter subsequently produced boys. One possible reason why the effect was significant only after a firstborn daughter is that male fetuses are notoriously sensitive to a wide range of malign forces during pregnancy. So the birthweight of boys is not as reliable an indicator of a mother’s intrinsic physical condition as that of girls.
Pawlowski believes his study shows the Trivers-Willard effect in action. He also suggests a possible mechanism, pointing out that at conception the ratio of males to females is 130:100, but that this drops to 106:100 by the time of birth because boys are more likely to miscarry than girls. “I believe women in poorer biological condition are more prone to miscarry male fetuses,” he says.
While Pawlowski gives a leading role to mothers, Valerie Grant in Auckland goes further, claiming sex determination is a one-woman show. “People everywhere have it ingrained in them that the father’s big contribution to the reproductive process is the sex of the offspring,” she says. Four decades of research have convinced her otherwise. It sounds odd, but Grant says she can predict whether a woman is more likely to have a boy or girl depending on her personality.
She asks prospective mothers to fill out a questionnaire that measures dominance.
The test, which is also available for a ÂŁ3 fee on the Internet (), consists of 64 adjectives such as proud, free, bored, awed and arrogant, which women tick if the word applies to them. Sprinkled among them are 13 words linked with high dominance and women are scored on how many of these they tick. The average score is about 3.
The test is impressively accurate for women with relatively high or low figures, although it is less precise for the majority with intermediate scores. Grant’s records show, for example, that pregnant women who score eight or above have an 80 per cent chance of having a boy.
The key is testosterone, she says. And her latest study with colleague John France confirms her assumption that women who score highly on the dominance test also have the highest levels of the male hormone (Biological Psychology, vol 58, p 41).
Grant admits that other scientists are sceptical of her work, despite the consistency of her published research over the years. “People are simply not persuaded by psychological tests,” she says. “The whole idea will only be taken seriously when I can demonstrate the reproductive basis.”
So she’s working on a theory. “My working hypothesis is that the female produces an ovum each cycle, already adapted to receive a spermatozoon bearing an X or a Y chromosome,” she says. Grant admits she doesn’t know the mechanism, but points to studies suggesting that testosterone in the ovaries regulates egg development. She speculates that high testosterone levels somehow prime developing eggs to be receptive to fertilisation by Y sperm, and low testosterone primes them for X sperm.
When it comes to the reason women manipulate the sex of their offspring in this way, Grant does not entirely subscribe to the Trivers-Willard effect. She claims it allows mothers to have babies of the sex they are best suited to nurture. We know that women react differently to boys and girls, says Grant, and babies of different sexes respond best to different sorts of stimulus. So there would be an adaptive advantage if a mother could have a child of the sex that suits her temperament. And that child might in turn also be a more successful parent.
Grant’s theories have been endorsed to some extent by work by John Manning at Liverpool University, who reported in July that both women and men have more sons if they have a long ring finger compared with their index finger (Journal of Theoretical Biology, vol 217, in press). What could possibly be the connection with finger length? Manning has previously shown that this physical trait is linked to high testosterone levels in the womb. “Grant may be right that emotionally independent and dominant women tend to have more boys,” says Manning. But he’s not convinced that this means mothers are in control of the sex of their children.
Of course, there’s no reason why both males and females might not “try” to manipulate the likely sex of their children, but each using different mechanisms. Such competing, selfish genes might result in an evolutionary “arms race” which continues until the benefits of influencing an offspring’s sex are outweighed by the harm caused by interfering with crucial reproductive mechanisms.
But Grant, at least, is convinced that women have the starring role. She points out that, compared with males, females have such a huge commitment to the conception, nurturing and raising of offspring that they’d be expected to evolve the ability to produce babies of the sex that suits them. She says: “I think the evidence that the male has anything to do with it is very flimsy.”
- Sex Ratios, edited by Ian Hardy,Cambridge University Press (2002)